1. Field of the Invention
The present invention relates to a wire bonding method, and more particularly to a method for forming a ball at the end of a bonding wire.
2. Prior Art
Various wire bonding methods have been proposed, and FIG. 3 illustrates one of the currently most commonly used methods.
In FIG. 3, first, in step (a), a ball 3a is formed on a wire 3 extending from the lower end of a capillary 4 by means of a spark discharge produced by an electric torch 5 while a clamper 6 is in a closed state to hold the wire 3. Afterward, the electric torch 5 is withdrawn in the direction of arrow T; and at the same time, the damper 6 is opened. When the damper 6 is opened, the wire 3 is raised by the back tension which has been applied to the wire 3, and the ball 3a is pressed against the chamfer (not shown) which is formed at the lower end of the capillary 4.
Next, in step (b), the capillary 4 is moved horizontally to a position above the first bonding point 1a. Then, in step (c), the capillary 4 is lowered so that the ball 3a on the tip end of the wire 3 is pressed against the first bonding point 1a; and an ultrasonic vibration is applied to the capillary 4 by a horn (not shown) on which the capillary 4 is provided. The ball 3a is thus bonded to the first bonding point 1a.
Subsequently, in step (d), the capillary 4 is raised, and in step (e), the capillary 4 is moved to a position above the second bonding point 2a. Next, in step (f), the capillary 4 is lowered so that the wire 3 is pressed against the second bonding point 2a, and an ultrasonic vibration is applied to the capillary 4 by the horn, thus bonding the wire 3 to the second bonding point 2a. Afterward, the capillary 4 is raised to a predetermined position, and the damper 6 is closed; and then the capillary 4 and damper 6 are both raised together so that the wire 3 is cut in step (g). When the wire 3 is thus cut, a tail length L required for forming the next ball remains extending from the lower end of the capillary 4.
The bonding method as described above is disclosed in, for instance, Japanese Pre-Examination Patent Publication (Kokai) Nos. S57-87143, S63-77131 and S64-17433. Furthermore, the capillaries referred to above are disclosed in, for instance, Japanese Examined Utility Model Publication (Kokoku) No. H1-42349 and Japanese Examined Patent Publication (Kokoku) No. H3-780.
In the prior art described above, the tail length L is longer than the length that is actually required for forming the ball 3a, and the size of the ball 3a is controlled by means of the output value and output time of the discharge current or discharge voltage. However, in the prior art method, especially when a small ball 3a is to be formed, it is necessary to reduce the discharge current or lower the discharge voltage so as to form the ball 3a in a shorter time. As a result, the discharge tends to be unstable due to external factors (including temperature and the gap between the electric torch 5 and tip end of the wire 3). Furthermore, an operation is required to open the damper 6 so that the damper 6 pulls up the wire 3 by way of back tension after the ball formation in order to prevent the center deviation of the ball 3a.